The present invention relates to a manufacturing method of a liquid crystal display (LCD) device. More particularly, the present invention relates to a method in which a terminal is formed on a lower substrate of the LCD device, and in which the terminal is then connected to a common electrode provided on a upper substrate to thereby electrically connect the upper and lower substrates.
In a conventionally known thin-film-transistor liquid crystal display, a plurality of liquid crystal cells are provided in a matrix form on the central portion of a lower glass substrate (hereinafter referred to as lower substrate). The liquid crystal cells include a pixel electrode which is an active area. A thin film transistor (TFT) is provided in each of the liquid crystal cells and are each used as a switching element for driving the respective liquid crystal cells. The TFTs are connected to a display signal electrode and a scanning signal electrode. Bonding pads for connection to an external driving circuit are formed around the periphery of the liquid crystal cells on the lower substrate of the LCD device. The display signal electrodes and the scanning signal electrodes of the TFTs are tied to the bonding pads so as to be electrically driven by external driving circuitry through the bonding pads.
A common electrode is formed on the overall surface of the upper substrate. In operation, an electric field is applied to the common electrode and to the opposing pixel electrode formed within the respective liquid crystal cells of the lower substrate, thereby implementing the electro-optical property of liquid crystal.
Since, when the LCD device is assembled, the distance between the upper and lower substrates is only several microns with the liquid crystal cells interposed therebetween, it is unsuitable to form the terminals of the electrodes (for instance, the pixel electrode, the gate electrode and the common electrode) of the upper and lower substrates on their own substrate to drawn them out for external connection. For this reason, the terminal of the common electrode formed on the upper substrate is formed on the lower substrate with an electrical connection between the common electrode on the upper substrate and the terminal formed on the lower substrate. In this manner, the electrode terminals can all be gathered on the lower substrate for external connection thereto.
The contact area for the terminal on the lower substrate is formed wider than that of the contact area on the upper substrate. Since the common electrode is formed on the overall surface of the upper substrate, even just a few terminals for the common electrode on the upper substrate can be drawn out via the lower substrate.
In practice, a lead wire of the common electrode is formed on the upper substrate of the liquid crystal display device and extended to the periphery of the upper substrate so as not to be superimposed with the scanning or display signal electrodes of the lower substrate. For easy contact with the lower substrate, a relatively wide contact portion of the above lead wire is formed on the periphery of the upper substrate of the LCD device. A contact portion of an electrode is formed on the lower substrate, opposite the contact portion of the common electrode formed on the inner surface of the upper substrate.
When the LCD device is assembled, the contact area of the lead wire of the common electrode of the upper substrate is electrically shorted to the contact area of the electrode on the lower substrate, with an externally accessible (10Utermfzlthe common electrode formed on the lower substrate.
FIG. 1 is a flowchart of the conventional manufacturing process of a liquid crystal display device using a conventional TFT structure. Referring to FIG. 1, the lower substrate of the LCD device is formed by first forming a bonding pad which will be used for electrically connecting the common electrode with an external driving circuit. The bonding pad is formed of a conductive material, e.g., chromium, on a transparent glass substrate. An electrode forming a contact portion on the lower substrate is also formed in an appropriate portion of the periphery of the lower substrate.
A conventional TFT is then formed on the lower substrate within an active area where the liquid crystal cells of the liquid crystal display device are to be formed. Pixel electrodes of ITO are also provided to form respective pixel areas of the LCD device.
Next, a passivation layer is formed on the overall surface of the lower substrate. Thereafter, portions of the passivation layer are removed to expose the bonding pads and contact portions of the electrode on the lower substrate. Conventional photolithography methods are used to remove the select portions of the passivation layer. In this process, the active area of the liquid crystal display device is protected by a photo-sensitive layer, and the passivation layer on the bonding pad and on the contact portion of the electrode of the lower substrate is selectively etched.
The photo-sensitive layer is removed by a conventionally known ashing process, and a liquid crystal sealant is used. Regarding the liquid crystal sealant, any conventionally known liquid crystal sealant may be used. If necessary, a spacer is equipped in the liquid crystal cell. Then, a conductive paste is formed between the contact areas of the upper and lower substrate.
The upper substrate of the liquid crystal display device is provided by forming an oblique layer on a transparent glass substrate so as to delimit the respective pixel areas of the liquid crystal display device. If desirable, a color filter is provided thereon. A transparent common electrode is thereafter formed on the overall surface of the resultant structure of the upper substrate.
Subsequently, the upper and lower substrates are assembled and hardened by heat. Liquid crystal is injected in the respective liquid crystal cells. The liquid crystal injection hole is sealed with any conventionally known hardening agent.
The liquid crystal cell is functionally tested, and a module process is performed such as attaching a polarizing plate and a light reflecting plate to the liquid crystal cell, and connecting an external driving circuit.
In the conventional manufacturing method for the liquid crystal display device, in order to remove the passivation layer formed on the bonding pad and on the contact portion of the upper and lower substrates, an additional photolithography process is performed. This additional photolithography process increases the production cost as well as the processing time for the LCD device.
Another conventional technique for removing the passivation layer on the bonding pad and on the contact portion of the upper and lower substrates on the lower substrate has been suggested. In this technique, the passivation layer is deposited after a shadow mask is formed on the bonding pad and on the contact portion of the upper and lower substrates. However, the shadow mask creates flaws in the pattern when the shadow mask is attached. Furthermore, since the passivation layer is deposited to a thickness of hundreds of .ANG., an overall etching should be re-performed. The specific process conditions of the re-performance of the etching is difficult to control.
Therefore, in the conventionally known methods, the additional photolithography steps add to the processing cost and time for manufacturing of the LCD device.